The present invention relates to a method for producing a nitride semiconductor light-emitting device, and more particularly to a method for producing a nitride semiconductor light-emitting device improved to prevent cracking and obtain nitride semiconductors and nitride semiconductor laser devices at a high yield rate. The present invention also relates to a nitride semiconductor light-emitting device obtained by such a method.
By using nitride semiconductor crystals represented by GaN, AlN, InN, and mixed crystals thereof, semiconductor laser devices that oscillate in the ultraviolet-visible region are being produced. As the substrate, a GaN substrate is often used and being studied with vigor in associated research institutions. Currently, the yield rate of semiconductor laser devices (e.g., the rate of good products obtained from one wafer) is significantly low, and improvement is highly necessary for cost reduction and the like. One of the causes that have kept the yield rate low is the occurrence of cracking.
As a technique to reduce occurrence of cracking, a method of using a processed substrate is proposed as shown in FIG. 6 (see, for example, Japanese Patent Application Publication No. 2002-246698). Referring to FIG. 6(A), processed substrate 61 (nitride semiconductor substrate) includes grooves 17 that are concave portions each processed in a stripe shape on the substrate surface. Over hills 18, which are convex portions each processed in a stripe shape, a light-emitting device made of nitride semiconductor is formed. It is known that by using such processed substrate 61 concaves are included in the surfaces of the semiconductor films that have been grown, thereby inhibiting occurrence of cracking in the light-emitting device.
It is also known that cracking is reduced by using a processed substrate shown in FIG. 6(B). This processed substrate has different kind substrate 62 (sapphire substrate, SiC substrate, Si substrate, GaAs substrate, or the like) and nitride semiconductor layer 64 formed over different kind substrate 62 via buffer layer 63 (nitride semiconductor layer (of low temperature or high temperature)). On the surface of nitride semiconductor layer 64, grooves 17 are formed each in a stripe shape. Over hills 18, which are convex portions each processed in a stripe shape, a light-emitting device made of nitride semiconductor is formed.
However, when the above-described processed substrate was used and nitride compound semiconductors were grown over the substrate by the MOCVD (Metal Organic Vapor Deposition) method or the like to prepare a semiconductor laser device, occurrence of cracking was prevented, but no great improvement in the yield rate was obtained.
The prevent inventors carried out an extensive study to analyze the cause of why the yield rate did not improve greatly. As a result, as shown in FIG. 7, it has been found that nitride semiconductor layers (those in the areas 130 shown in FIG. 7) accumulated so as to cover from the side surfaces of the groove portions (the portions where grooves portions 17 and hill portions 18 were in contact) to hill portions 18 have undulating-shaped surface morphologies 140 and are grown much thicker than at the center portions of hill portions 18. (This growth will be hereinafter referred to as abnormal growth.) In addition, the sizes and thicknesses of abnormal growth portions 130, caused by abnormal growth, were different between adjacent groove portions 17, and along the stripe of the groove portion 17 even if it was the same groove portion 17. It has been found that if abnormal growth portion 130 is not formed uniformly in each groove portion 17 as described above, the formation of undulating-shaped surface morphology 140 is promoted in the vicinity of the edges of groove portion 17, and that the promoted surface morphology 140 undermines the thickness uniformity and flatness of the nitride semiconductor layers in the vicinity of the center of hill portion 18. It is considered that occurrence of such a phenomenon is because when grooves 17 are formed on the nitride semiconductor substrate by dry etching or wet etching, the edge portions of the sides of groove portions 17 cannot be made uniform and take various shapes.
Thus, if concaves (grooves) remain on a nitride compound semiconductor, the concaves cause deterioration of the flatness of the film. It is considered that the deteriorated flatness in turn causes variation of thickness of each layer of the device and chip-to-chip fluctuation of characteristics, resulting in a deteriorated yield rate. In other words, in order to improve the yield rate, it is necessary to improve film flatness as well as reducing occurrence of cracking.